1. Field of the Invention
The present invention relates to a mine roof bolt anchored in a bore hole by mechanical anchoring, resin bonding, or both and more particularly to a mine roof bolt bearing a segmented resin compression layer that exerts a compressive force on resin within a bore hole.
2. Description of Related Art
The roof of a mine conventionally is supported by tensioning the roof with 4 to 6 feet long steel bolts inserted into bore holes drilled in the mine roof that reinforce the unsupported rock formation above the mine roof. The end of the mine roof bolt may be anchored mechanically to the rock formation by engagement of an expansion assembly on the end of the mine roof bolt with the rock formation. Alternatively, the mine roof bolt may be adhesively bonded to the rock formation with a resin bonding material inserted into the bore hole. Alternatively, a combination of mechanical anchoring and resin bonding can be employed by using both an expansion assembly and resin bonding material.
A mechanically anchored mine roof bolt typically includes an expansion assembly threaded onto one end of the bolt shaft and a drive head for rotating the bolt. A mine roof plate is positioned between the drive head and the mine roof surface. The expansion assembly generally includes a multi-prong shell supported by a threaded ring and a plug threaded onto the end of the bolt. When the prongs of the shell engage with rock surrounding a bore hole, and the bolt is rotated about its longitudinal axis, the plug threads downwardly on the shaft to expand the shell into tight engagement with the rock thereby placing the bolt in tension between the expansion assembly and the mine roof surface.
When resin bonding material is used, it penetrates the surrounding rock formation to adhesively unite the rock strata and to firmly hold the roof bolt within the bore hole. Resin is typically inserted into the mine roof bore hole in the form of a two component plastic cartridge having one component containing a curable resin composition and another component containing a curing agent (catalyst). The two component resin cartridge is inserted into the blind end of the bore hole and the mine roof bolt is inserted into the bore hole such that the end of the mine roof bolt ruptures the two component resin cartridge. Upon rotation of the mine roof bolt about its longitudinal axis, the compartments within the resin cartridge are shredded and the components are mixed. The resin mixture fills the annular area between the bore hole wall and the shaft of the mine roof bolt. The mixed resin cures and binds the mine roof bolt to the surrounding rock. The typical diameter of a mine roof bore hole is one inch. Mine roof bolts anchored with resin bonding are often ¾ inch in diameter, and more recently ⅝ inch in diameter. The mine roof bolt is generally centered within the bore hole creating a circular annulus that becomes filled with bonding resin. The larger diameter bolts (¾ inch) offer performance advantages over ⅝ inch bolts in that the annulus provided between the bore hole wall and a ¾ inch bolt is smaller than that of smaller diameter bolts. A smaller annulus provided between the bolt and the bore hole wall improves mixing of the resin and catalyst in the annulus. In addition, when the resin cartridge is shredded upon insertion of the mine roof bolt and rotation thereof in an annulus larger than ⅛ inch (as for mine roof bolts having less than ¾ inch diameter installed in one inch bore holes), the shredded cartridge can interfere with the resin and catalyst mixing. Poor mixing results in an inferior cured resin and results in poor bond strength between the bolt and bore hole wall. This phenomenon of “glove fingering” occurs when the plastic film that forms the cartridge lodges in the bore hole proximate the surrounding rock thereby interrupting the mechanical interlock desired between the resin and bore hole wall. In addition, the larger annulus created by using a ⅝ inch bolt in a one inch bore hole requires more resin to bond the bolt to the rock than does a larger diameter bolt, thereby adding to the cost of installing a smaller diameter bolt. While one solution would be to proportionally reduce the size of the bore hole to less than one inch, this is not practicable. The mine roof drilling equipment in use is conventionally produced for drilling one inch bore holes. Moreover, there are significant technical difficulties in drilling small diameter bore holes in mine roofs.
Despite these drawbacks of using mine roof bolts having a diameter of less than ¾ inch, the popularity of smaller diameter mine roof bolts is increasing. A ⅝ inch bolt is lighter and easier to use than a ¾ inch bolt and can be produced at lower cost. One solution for overcoming the need for extra resin and avoiding the glove fingering problem of smaller diameter bolts installed in one inch bore holes has been provided in a proposed mining bolt which includes an elongated rod that forms the main structure of the mine roof bolt as disclosed in U.S. Patent Application Publication No. 2005/0134104. A portion of the rod in between a drive head and the end of the bolt is coated with a layer of material having a lower specific gravity than the rod, such as a polymer. The polymeric coating layer may have external texturing which can help with mixing of resin in the mine roof bore hole. The coating on the mine roof bolt also helps to fill some of the annulus at a minimal increase in weight to the bolt and minimizes the amount of resin that is required for bonding the bolt to rock strata. This coated mine roof bolt can be produced from a ⅝ inch metal rod with a polymeric coating layer about 1/16 inch thick. The coated mine roof bolt uses only resin bonding to anchor the mine roof bolt to a rock formation.
However, the combination of both mechanical anchoring and resin bonding of mine roof bolts has been found to provide superior mine roof control. A mine roof bolt having an expansion assembly with expansion shell and plug is held against the surface of a mine roof by a plate. Rotation of the bolt mixes the resin components and expands the expansion shell. The resin mixture surrounds the expansion assembly and several feet of the mine roof bolt. Upon hardening of the resin mixture, the bolt is anchored to the rock strata by the resin and the expansion assembly. In some mine roof bolts that are anchored by a combination of resin bonding and expansion assembly anchoring, a device is used to delay relative rotation between the expansion assembly and the mine roof bolt until the resin is hardened so that the bolt can be tensioned after the resin begins to harden. An anti-rotation device prevents relative rotation between the plug of an expansion assembly and the bolt so that the plug does not thread down the bolt during mixing of the resin components. One suitable anti-rotation device is a shear pin extending through the plug. The resin components are thoroughly mixed before the shell of the expansion assembly is expanded. The end of the bolt abuts the pin to prevent initial downward movement of the plug on the bolt during rotation of the bolt to effect mixing of the resin components. Once the resin begins to set, the force on the shear pin exceeds its strength and continued rotation of the bolt shears through the pin and allows the plug to advance downwardly on the bolt to expand the shell of the expansion assembly outwardly to grip the bore hole wall.
For mine roof bolts that are anchored using a combination of a mechanical anchor and resin bonding and for coated mining bolts that are anchored with resin, the resin is desirably maintained in an upper region of the bore hole. However, retention of the resin adjacent the upper portion of the mine roof bolt is problematic. One solution has been to include a resin retaining washer at a position intermediate the end of the mine roof bolt and the mine roof for restricting the annular area in which the resin may flow. The upward thrust of a mine roof bolt bearing a resin retaining washer can exert a hydraulic force on the resin to confine it within the restricted annular area at the end of the mine roof bolt and forcibly drive the resin into the cracks and crevices on the inside of the bore hole and into the surrounding rock formation to more solidly lock the mine roof bolt within the rock formation. However, such resin retaining washers are limited in their ability to block resin from flowing downwardly along the bolt. While a resin retaining washer can withstand the hydraulic pressure created when the mine roof bolt shreds the resin capsule, nothing on the mine roof bolt urges the resin back upwardly into the bore hole.
Accordingly, a need remains for a mine roof bolt which utilizes mechanical anchoring or resin bonding, or both, to anchor the mine roof bolt in a bore hole (particularly for a small diameter mine roof bolt such as ⅝ inch) where the resin mixing and distribution is controlled by the bolt.